Department of Chemistry, University of Kansas, Lawrence, KS 66045.
Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973-5000.
Proc Natl Acad Sci U S A. 2023 May 23;120(21):e2217189120. doi: 10.1073/pnas.2217189120. Epub 2023 May 15.
Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η-pentamethylcyclopentadiene (CpH) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by CpH, using CpRh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of CpRh(bpy) is [CpRh(H)(bpy)], an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(CpH)Rh(bpy)]. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related CpH complex can be involved in further reactivity, showing that [(CpH)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.
涉及有机金属配合物的质子化反应在氧化还原化学中普遍存在，通常会导致反应性金属氢化物的生成。然而，最近一些由η-五甲基环戊二烯基（Cp*）配体支持的有机金属物种被证明可以通过酸的直接质子转移或金属氢化物的互变异构化发生配体中心质子化，从而生成具有不常见的η-五甲基环戊二烯（CpH）配体的配合物。在这里，使用 CpRh(bpy) 作为分子模型（其中 bpy 是 2,2'-联吡啶），应用时间分辨脉冲辐射（PR）和停流光谱研究来检查导致配位 CpH 的配合物的基本电子和质子转移步骤的动力学和原子细节。停流测量与红外和紫外-可见检测相结合，揭示了 CpRh(bpy) 的初始质子化的唯一产物是[CpRh(H)(bpy)]，这是一种难以捉摸的氢化物配合物，这里对其进行了光谱和动力学表征。氢化物的互变异构导致[(CpH)Rh(bpy)]的干净形成。变温实验和同位素标记实验进一步证实了这一分配，提供了实验活化参数和对金属介导的氢化物-质子互变异构的机制见解。对第二个质子转移事件的光谱监测表明，氢化物和相关的 CpH 配合物都可以进一步参与反应，表明[(CpH)Rh]不一定是离线中间体，而是根据用于驱动催化的酸的强度，是氢逸出的活性参与者。在研究中识别出催化中质子化中间体的机制作用可以为设计由非无辜环戊二烯型配体支持的优化催化系统提供信息。
